1. Field
The applicant's disclosure relates generally to disk drives and more particularly to an improved method and apparatus for servo track writing disks used in a hard disk drive assembly.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
There are two general types of servo systems whereby the actuator is controlled utilizing servo information which has been recorded on one or more of the disks in the hard disk drive. The first such servo system is generally referred to as a dedicated servo system. With this servo system one disk surface is devoted to containing servo information. One of the read/write heads is then used to read the dedicated servo disk and to provide position information such that the positions of the other read/write heads in the hard disk assembly may be obtained. A second type of servo system is commonly known as an embedded sector servo system. With this servo system, rather than using a single disk surface to contain servo information, the servo information is stored in specific sectors across several or all of the disks in the hard disk drive assembly. Moreover, the servo information may be recorded either prior to assembly of the hard disk drive, or after assembly.
In either case, the servo information must be recorded, or servo written, onto the disk surface using a highly precise encoding system. The current method for servo writing utilizes a mechanical pushpin. With this system, a mechanical pushpin is attached to a master actuator arm at one end and extends into the hard drive through an access slot on the other end. The master actuator arm is controlled using a positioner in a closed loop with a highly accurate encoder. In addition, a clock head is directed to write a clock track onto a disk to serve as a timing reference during the servo writing process.
This process suffers from problems associated with non-repetitive run-out (“NRRO”), disk flutter and motor rocking, all of which serves to degrade the accuracy of the position information written during the servo writing process. Moreover, the use of a positioner/encoder greatly increases the expense associated with the servo writing process.
A second method of servo writing that has been used is the traditional offline servo track writing method. With this method, each disk is servo written using an offline servo track writer before being installed into a hard disk drive assembly. While this method has the benefit of increased accuracy due to the fact that it does not use a pushpin, such traditional systems are still dependent on the use of a positioner and precision encoder, thereby dramatically increasing the expense associated with the use of this method.
Another issue with traditional offline track writing is the error introduced by the bearing assembly of the spindle motor used to rotate the disk during the writing process. Traditional ball bearing assemblies suffer from mechanical drawbacks, such as excessive wear and run out. Increasing the data density of a disk can be limited by NRRO. The spindle bearing is the primary contributor to increasing NRRO. Moreover, improved NRRO improves seek time and ability to track follow for a servo writer. To alleviate these problems, air-bearing spindles have been used for traditional offline track writing purposes. Air-bearing spindles are able to reduce NRRO by reducing the amount of surface-to-surface contact in the spindle. However, air-bearing spindles dramatically increase the expense of using an offline servo writing system.
Therefore, there is a need in the art for an improved method and apparatus for servo writing that does not suffer from these drawbacks.